1. Field of the Invention
This invention relates to methods for fluxless soldering.
2. Description of the Related Art
Fabrication and assembly of electronic circuits requires that certain components of the circuit be in electrical contact with each other. Thus, integrated circuits (chips) or other components such as resistors, capacitors, etc., are mounted on substrates such as printed wiring boards so that the chip and substrate make electrical contact and are held together with the substrate supporting the component.
Processes for connecting the components and the substrate generally involve soldering. The first step in most current soldering processes is a pre-cleaning and deoxidation of surface oxides, generally using a liquid flux material, such as a weak organic acid applied prior to soldering and activated during the soldering process (e.g., at 230.degree. C.) in a wave solder bath. Different flux materials are used in the pre-cleaning step to prepare the surface to be soldered by removing contaminants including metal oxides from the solder surface.
The second step in soldering involves solder reflow and/or reflow joining. This step can occur only after all oxides are removed from the solder surface because the oxides prevent the wetting of the two surfaces to be joined by the soldering reflow. When solder is heated it reflows and joins the surfaces which are in contact with solder.
The third step is post-soldering cleaning to remove the flux residue. This step is particularly difficult due to the small size of typical electronic components, and the difficulty of getting the cleaning agents to reach the minute areas between components.
Numerous investigators have attempted to eliminate the flux requirement for soldering. Fluxless soldering, or soldering without use of a liquid flux, replaces the first step with an alternate treatment, and can eliminate the third cleaning step. One example of fluxless soldering methods is the process of Dishon and Bobbio which utilizes plasma excitation using innocuous fluorinated gases, such as SF.sub.6 or CF.sub.4, to remove surface oxides from solder surfaces (U.S. Pat. No. 4,921,157). It is believed that these and other fluorinated gases of non-reactive substances were used as a source of plasma dissociated fluorine atoms, so that these atoms would react with and fluorinate the tin oxide (SnO.sub.2) surface layer on the solder. Once this reaction occurs, the solder surface tension is sufficiently lowered so that joining is possible. In this method, the joining or reflow may be done any time within two weeks of the fluorination.
Other more recent examples of fluxless soldering include wave soldering (U.S. Pat. No. 5,044,542 of Deambrosio), palladium enhanced fluxless soldering (U.S. Pat. No. 5,048,744 of Chang et al.), use of various inert gases (U.S. Pat. No. 5,139,193 of Todd), use of reducing agents such as lithium, calcium, strontium and cesium (U.S. Pat. No. 5,139,704 of Holland et al.), use of a laser beam (U.S. Pat. No. 5,164,566 of Spletter et al.), use of C-O-H-F (U.S. Pat. No. 5,071,486 of Chasteen), use of a heated reducing or non-reactive gas (U.S. Pat. No. 5,205,461 of Bickford et al.), and use of carbon-fluoride compositions (U.S. Pat. No. 5,380,557 of Spiro).
Unexpectedly, it has been found by the inventors herein that use of HF, having very strong internal bonds and not dissociating easily to yield free fluorine, is an efficient method of fluxless soldering and provides a surface layer on the solder which allows fluxless reflow.
In addition to using the method of the invention with solder on a surface, the method may be used for treatment of solder powder which is used in solder paste. Conventional solder paste is a composite of solder powder and a vehicle which contains solder flux where the vehicle acts as a suspending medium for the powder. Solder paste may be applied to components and circuit boards in a number of ways including screen printing. During reflow, the flux dissolves surface oxides on the powder to allow the powder particles (now liquid) to flow smoothly together and to the parts to be joined. As with other conventional soldering processes, the flux leaves a residue which should be cleaned after joining. The method of the invention also allows fluxless soldering with solder paste in which the solder powder has been pretreated using the methods described herein.
Other objects and advantages will be more fully apparent from the following disclosure and appended claims.